String galvanometer



May 15, 1934. J. D. DIGBY STRING GALVANOMETER Filed Nov. 30, 1929 l ..L| l.l

ATTORNEY Patented May 15, 1934 UNITED STATES PATENT OFFICE The Cambridge Instrum lining ent Company, Inc s- N. Y., a corporation of New York Application November 30, 1929, Serial No. 410,741 12 Claims. (Cl. 171-95) The present invention, while having also other and broader fields of usefulness, is more particularly related to string galvanometers.

A very efiicient Einthoven string galvanomoter i is disclosed, for example, in United States Patent No. 1,416,824, granted May 23, 1922. The string member of such a galvanometer is ordinarily constituted of a quartz or glass thread, very thin- 1y coated with gold or other metal to render it conductive. The conductive coating is so thin as not appreciably to affect the original properties of the quartz or glass thread. It results from this construction that the thermal expansivity of the finished string member is substantially the same as that of the quartz or glass, and is appreciably unaffected by the metal coating. The thermal expansivity of the string member is thus considerably diiferent from the base or frame support, usually of metal, upon which it is mounted. Changes in the temperature of the galvanometer, introduced by its high energizing current, and even, sometimes, by changes in the temperature of the room, thus cause changes in the degree of tautness of the string member and destroy the proper position 'of the string member relative to other parts of the apparatus. As the measurements obtained with such instruments depend for their accuracy upon the exceedingly slight, sidewise movement of the electro-conductive string member, in response to exceedingly weak changes in the strength of an electro-magnetic field, variations in the degree of tautness of the string member introduce inaccuracies which vitiate the whole measurement.

It is, accordingly, an object of the present invention to improve upon string galvanometers of the above-described character, to the end that they shall be of substatnially equal accuracy irrespective of temperature changes. Other and further objects will be explained hereinafter, and will be particularly pointed out in the appended claims.

The invention will be explained in connection with the accompanying drawing, in which Fig. 1 is a view of an eleetro-conductive string member with a support or base upon which it is mounted; Fig. 2 is a similar view of a modification; and Fig. 3 is a view, upon a smaller scale, of the string member and its support in position in a string galvanometer, shown in section.

A brief description of one type of string galvanometer will now be given to serve as a setting for an explanation of the features of novelty of the present invention. Thev galvanometer is provided with an electromagnet having slightly spaced pole pieces, shown in Fig. 3 at 60, 69. The energizing coils for the magnet are indicated at 41 and 42. The invention may, of course, be used also in other types of galvanometers, such as are illustrated, for example, in the abovenamed patent, or in United States Patent No. 1,702,650, granted February 19, 1929, the type shown in the drawing being for illustrative purposes only. The string member 23 is suspended in the electro-magnetic field produced in the restricted air gap between the slightly spaced pole pieces, and at the focus of light rays that proceed from a lamp or other source of light 62, through tubes 55 and 45, to a light-sensitive film in a photographic camera (not shown). The light from the lamp 62 is condensed by a condenser system 61, is focussed on the string member by a microscope 63, and is projected to the camera (not shown) by a lens system 64, '70. The, magnification may be from 20 to 1,000 times. In 15 electrocardiography, for example, it may be 600 times.

The string member 23 is shown in Fig. 1 as carried by a base, frame or support 1, and in Fig. 2 by a similar support 24. These supports an 1 and 24 may have a magnifying lens 21 for providing an enlarged view of the string member 23, and the space between the pole pieces in which it is disposed. One end of the string member is secured in Fig. 1 to a supporting head 19 and 35 the other to a supporting head 10, the former being stationary upon the frame 1 and the other adjustable. The adjustable supporting head 10 is fixed to a plate 53 of a slide 9 one end of which bears on a lug 54 of the frame 1 and the other inaplate l'lcarriedbyaluglsoftheiramel.

A spring 16, coiled about the slide 9, the plate 17 and a plate 56 of the slide 9, thru tending to urge the slide 9 yieldingly toward the right, as viewed in Fig. 1. A pin 13 spans the 5 plates 53 and 56 and serves as an anchorage for one end of a coil spring 11, the other end of which is secured to a button 12. The spring 11 thus tends to hold the slide 9 against the bearings 54 and 18. The plate 56 has a depending 1 o finger 14 that extends into a lug 15 of the irame 1 for guiding purposes. The adjustable head 10 is thus yieldingly but firmly held in adjusted position.

The adjustment of the movable head 10 is effected by means of a screw 3 that may be turned by a thumb wheel 2. A very small movement of the thumb wheel 2, resulting in a corresponding forward or backward movement of the screw 3, will cause a lever 5, through a thrust pin 4,

to pivot slightly in one direction or the other about a pivotal point '7. The degree of pivotal movement of the lever 5 may be adjustably limited by a stop screw 6, which prevents the lever 5 being thrown out of position. The pivotal movement of the lever 5 in one direction is communicated, by means of a thrust rod 8, to the slide 9, causing the latter to move to the left, as viewed in Fig. 1, carrying the head 10 with it. When the thumb wheel 2 is moved in the opposite direction, the spring 16 causes the head 10 to move toward the right, as viewed in Fig. 1. In this manner, it is possible to adjust the tension of the string member 23 so that the requisite or desired deflection may be obtained when astandard or test voltage is applied to the string-member terminals.

The string member 23, as before stated, may be constituted of a glass or quartz thread, very thinly coated with metal. The coefficient of linear thermal expansion of quartz is approximately 0.0052 10- per degree centigrade. The corresponding coefiicient of glass is 0.09 10- The base or support 1 must be strong and rigid and is,'therefore, commonly constituted of metal, such as cast brass, or cast iron. The corresponding' coeflicients of these two substances are 0.1875 10- and 0.106 10- respectively. Changes in temperature of the nature above designated cause different length expansions and contractions of the string and its supporting suspension, and this affects the sensitivity of the instrument. Changes in sensitivity of two or three hundred per cent often occur in actual use in as short a period of time as ten minutes. This, besides rendering the response of the string member less sensitive, also throws it out of focus.

and the errors thus introduced are often quite serious.

To compensate for this differential expansion between the string member 23 and the base 1, the string member is anchored to its support not directly, but indirectly, through bi-metallic com pensators. These are indicated in Fig. 1 at 20 and 22, in the form of U-shaped members bent from strip metal composed of two lamina. The inside lamina'50, for example, may be of invar or other low-expansivity material, and the outside lamina 52 of brass or other metal having a greater coefficient of thermal expansion. Other bi-metallic materials, such as are obtainable on the market, may also be used. Upon an increase in temperature, the frame or support will expand more than the string member 23. The length and the bends are so designed, however, that the U-shaped compensators 20 and 22 shall bend toward each other, and so prevent a change in tension in the string member 23. On a decrease in temperature, similarly, the members 20 and 22 simultaneously bend away from each other to produce the same result. As the numerical values before given indicate, the coeflicient of expansion of cast iron is not widely different from that of glass. If a castiron frame 24 and a glass string member 23 are used, as illustrated in Fig. 2, therefore, the degree of compensation needs only to be somewhat small.

The ends of the string member 23 may, in that event, be secured to two attachment elements 3''! and 30, one or both of which may be constituted of aluminum or some other material having a high coefficient of expansion. The coefficient of expansion of aluminum is 0.231 2 10- The lengths of the base 24, the string member 23 and the elements 3'? and 30 will be so designed as to produce the same linear distance between the points of attachment of the string member for any expected room temperature.

The arrangement shown in Fig. 2 is simpler than in Fig. 1, the better to illustrate the theory of the operation. The thumb screw 2 is here shown provided with a worm gear 25 meshing with a worm wheel 26 that is mounted to rotate with a threaded rod 27. The screw 27 threads in and out through the lug 28 of the base. or support 24. The head 39, that carries the attachment element 37, is fixed to the frame or support 24, as is the head 19 of Fig. 1, but so is, in this case, the other head 34. A blade spring 31 is, however, carried by the head 34, being held in place by a bolt 35 and a nut 36. The attachment element 30 is threadedly extended through the spring 31 for adjustment purposes. The spring 31 maintains an insulating portion 29 of the attachment member 30 tightly in contact with the free end of the screw 27. To facilitate this action, the said free end of the screw 2'7 is hollowed out, the better to receive the insulating member 29. I

Further modifications will also occur to persons skilled in the art, and all such are considered to fall within the spirit and scope of the invention, as

defined in the appended claims.

What is claimed is:

1. In a string galvanometer, a magnet having opposed slightly spaced pole pieces, a support, a string member having a body constituted of nonelectro-conductive material having an exceedingly small temperature coeflicient and stretched in a straight line lengthwise of itself on the support with a portion of the string member positioned in the space between the pole pieces, the body of the string member having a very thin metal coating to render it electro-conductive, the,

string member and the support having different coefiicients of thermal expansion, and means for compensating for the difference in the said 00- effloients of thermal expansion so as to maintain I the string member stretched in the said straight line lengthwise of itself on the support with the same degree of tautness irrespective of temperature changes.

2; In a string galvanometer, a magnet having opposed slightly spaced pole pieces, a support, a string member having a body constituted of nonelectro-conductive material having an exceedingly small temperature coefficient and stretched in a straight line lengthwise of itself on the support with.a portion of the string member positioned in the space between the pole pieces, the body of the string member having a very thin metal coating to render it electro-conductive, the string member and the support having different coefficients of thermal expansion, and a compensator disposed between the string member and the support for compensating for the difference in the said coefficients of thermal expansion so as to maintain the string member stretched n the said straight line lengthwise of itself on the support with the same degree of tautness irrespective of temperature changes.

3. In a string galvanometer, a magnet having opposed slightly spaced pole pieces, two anchorage members respectively on opposite sides of the space between the pole pieces, a string member having a body constituted of non-electro-conductive material having an exceedingly small temperature coefficient and the ends of which are carried by the anchorage members so thatthe string member is stretched in a straight line lengthwise of itself between the anchorage members with a portion of the string member positioned in the space between the pole pieces transversely to the path of the magnetic flux between the pole pieces, the body of the string member having a very thin-metal coating to render it electro-conductive, means for producing a beam of light traversing'the space between the pole pieces, means for receiving the shadow produced by the string member in the beam of light, and means for maintaining the string member stretched in the said straight'line lengthwise of itself between the anchorage members with the same degree of tautness irrespective of temperature changes.

4. The combination with an indicating device having a tensioned movable element suspended in a constant magnetic field, of a support for said element, said support responsive to temperature changes in such manner as to maintain constant the original tension in said movable element.

5. In combination with an indicating device having means for producing a steady magnetic field and a tensioned movable element suspended in said field, of means forming a support for said movable element for compensating for variations in the frequency of mechanical resonance of said element caused by temperature variations, said means being capable of adjustment as to its degree of compensation for particular temperature changes. a a

6. The combination with an indicating device having a tensioned movable element suspended in a magnetic field, of a support for said element, said element being supported at its ends by the support and stretched in a straight line lengthwise of itself between its ends, the string member and the support having different coefficients of thermal expansion, and means exerting a force at one of the ends of the string member to compensate for the difference in the said coefficients of thermal expansion so as to maintain the string member stretched in the same straight line lengthwise of itself on the support with the same degree of tautness irrespective of temperature changes.

7. In a string galvanometer, a magnet having opposed slightly spaced pole pieces, a support, a string member supported at its ends by the support and having a body constituted of non-electroconductive material having an exceedingly small temperature coefficient and stretched in a straight line lengthwise of itself between its ends with a portion of the string member positioned in the space between the pole pieces, the body of the string member having a very thin metal coating to render it electroconductive, the string mem her and the support having different coefficients of thermal expansion, and means exerting aforce at one of the ends of the string member to compensate for the difference in the said coefllcients of thermal expansion so as to maintain the string member stretched in the said straight line lengthwise of itself on the support with the same degree of tautness irrespective of temperature changes.

8. In a string galvanometer, a magnet having opposed slightly spaced pole pieces, a support, a string member supported at its ends by the support and having a body constituted of non-electroconductive material having an exceedingly small temperature coefiicient and stretched in a straight line lengthwise of itself between its ends with a portion of the string member positioned in the space between the pole pieces, the body of the string member having a very thin metal coating to render it electroconductive. the string member and the support having different coefficients of thermal expansion, and a compensator coacting between one of the ends of the string member and the support to compensate for the difference in the said coefficients of thermal expansion so as to maintain the string member stretched in the said straight line lengthwise of itself on the support with the same degree of tautness irrespective of temperature changes.

9. In a string galvanometer, a magnet having opposed slightly spaced pole pieces, a support having two anchorage members respectively on opposite sides of the space between the pole pieces, a string member having a body constituted of nonelectroconductive material having an exceedingly small temperature coefficient and the ends of which are carried by the anchorage members so that the string member is stretched in a straight line lengthwise of itself between the anchorage members with a portion of the string member positioned in the space between the pole pieces transversely to the path of the magnetic flux between the pole pieces, the body of the string member having a very thin metal coating to render it electro-conductive, the string member and the sup port having different coeflicients of thermal ex pansion, and a compensator coacting between one of the ends of the string member and the support to compensate for the difference in the said coefficients of thermal expansion so as to maintain the string member stretched in the said straight line lengthwise of itself on the support with the same degree of tautness irrespective of temperature changes.

10. In a string galvanometer, a magnet having opposed slightly spaced pole pieces, a support having two anchorage members respectively on opposite sides of the space between the pole pieces, one of the anchorage members being stationary on the support and the other anchorage member being adjustable on the support, a string member having a body constituted of non-electroconductive material having an exceedingly small temperature coefficient and the ends of which are carried by the anchorage members so that the string member is stretched in a straight line lengthwise of itself between the anchorage members with a portion of the string member positioned in the space between the pole pieces transversely to the path of the magnetic flux between the pole pieces, the body of the string member having a very thin metal coating to render it electro-conductive, the string member-and the support having different coefllcients of thermal expansion, and a compensator coacting between one of the ends of the, string member and the support to compensate for the difference in the said coeflicients of thermal expansion so as to maintain the string member stretched in the said straight line lengthwise of itself on the support with the same degree of tautness irrespective of temperature changes.

11. In a string galvanometer, a magnet having opposed slightly spaced pole pieces, a support having an anchorage member, a string member one end of which is secured to the support and the other end of which is secured to the anchorage member, the string member and the support having different coeflicients of thermal expansion, and the anchorage member having a coefiicient of thermal expansion of value to compensate for the difference in the two first-named coeificients of expansion so as to maintain the string member stretched in the said straight line lengthwise of itself on the support with the same degree of tautness irrespective of temperature changes.

12. In a string galvanometer, a magnet having opposed slightly spaced pole pieces, a support having an anchorage member, a string member one end of which is secured to the support and the other end of which is secured to the anchorage member, the string member and the support hav- 

